Early reactive power compensators used a thyristor-controlled inductor and a fixed or mechanically switched capacitor network to provide power factor correction for industrial loads. Later VAR generators were designed using thyristor-switched capacitors along with thyristor-controlled inductors, e.g., see U.S. Pat. No. 4,472,674 entitled "Method of Static Reactive Power Compensation" and U.S. Pat. No. 4,719,402 entitled "VAR Compensator System With Minimal Standby Losses."
A thyristor-switched capacitive (TSC) network indicated generally at 11 is shown in FIG. 1. Voltage V.sub.S from an AC electrical power supply network (not shown) is supplied over an AC transmission line 10. The thyristor-switched capacitive network 11 is connected to the transmission line 10, e.g. across one phase of the AC power supply network, and includes a capacitor 12 connected at one terminal to the AC transmission line 10 and at the other terminal to a surge inductor 14. Surge inductor 14 is also connected to thyristor switch 16 having antiparallel thyristors 18 and 22 which are controlled by externally generated firing signals 20 and 24, respectively.
Capacitor voltage V.sub.C, thyristor voltage V.sub.T, and thyristor current I.sub.T waveforms which illustrate operation of TSC network 11 are shown in FIG. 2 in normalized, per-unit (p.u.) values. Thyristor switches 18 and 22 are normally fired to conduct current in response to a VAR demand signal when the capacitor voltage V.sub.C and the AC network voltage V.sub.S (waveform not shown) are equal, i.e. when the voltage V.sub.T across the thyristor switches 18 and 22 is zero. The waveform shows that before time (-0.02) seconds, thyristor 16 is switched on, thyristor current I.sub.T flows, and the thyristor voltage V.sub.T equals zero. The thyristor current I.sub.T leads the capacitor voltage V.sub.C by ninety degrees. At time (0.00) seconds, the reactive power demand is removed turning off the thyristor causing I.sub.T to become zero. At this instant in time, the capacitor voltage V.sub.C is equal to the negative peak of the AC network voltage V.sub.S normalized to a value of -1.0 p.u. Typical absolute values of V.sub.S would be in the range of 5,000 to 50,000 volts. Since the thyristor 16 is disconnected and no current flows, the capacitor 12 remains charged to that voltage V.sub.C.
The voltage across thyristor switch 16 V.sub.T is the difference between the applied AC system voltage V.sub.S and the essentially DC capacitor voltage V.sub.C. This difference (V.sub.T) reaches a maximum value of twice the peak AC voltage, i.e. 2.0 p.u. once each cycle. Thus, the thyristor switch 16 must be rated to block this maximum value voltage. In addition, the AC network voltage V.sub.S may transiently increase considerably above its peak values to excessively high voltage levels. Should the capacitor 12 be disconnected at these higher voltage levels, both the capacitor 12 and thyristors 18 and 22 would be subjected to more than twice the peak AC voltage. Accordingly, both components must be designed and constructed with adequate insulation, etc. for those voltage levels adding considerable expense to component and system costs.
One partial solution is to connect a voltage limiting device such as a metal oxide varistor (MOV) across the capacitor 12, shown as varistor 23, and across the surge inductor 14 and thyristor switch 16, shown as varistor 25. Varistors have a high resistance at voltages below a predetermined value and a relatively low resistance at voltages above a predetermined value, and as a result, they protect against transient high voltage peaks. Exemplary protective circuits using MOVs are taught in U.S. Pat. No. 4,475,139 entitled "Thyristor-switched Capacitor Apparatus" and U.S. Pat. No. 4,636,910 entitled "Varistor Over Voltage Protection System With Temperature Systems." However, these systems using MOV devices still suffer from the serious drawback that the capacitor is charged to a voltage which causes the thyristor switch to see two times the normal peak voltage of the system.
Other over voltage protection systems inhibit disconnection of the capacitor 12 under high AC network voltage conditions by keeping the thyristor switch conducting. The drawback with this approach is that the connected capacitor increases the already high network voltage and may also create a dangerous oscillatory condition in the AC network which could further aggravate overvoltage problems. Thus, the voltage rating of the thyristor switch 16 must again be increased.
U.S. Pat. No. 4,571,535 entitled "VAR compensator Having Control Discharge Of thyristor-switched Capacitors" discloses a capacitor connected in series with two "half" thyristor switches where each half switch includes back-to-back thyristor switch pairs. Connected across each half-switch is a nonlinear clamping device, e.g. an MOV. When the thyristor switch is turned off, one of the two thyristor half-switches is kept on to effectively shunt its associated MOV so that the varistor for the off half-switch discharges the capacitor to a level determined by the breakdown voltage of the varistor connected across the nonconducting half-switch. The MOV in parallel with the nonconducting half-switch conducts the capacitor discharge current as long as the voltage exceeds its breakdown voltage with the conducting half-switch shunting the discharge current. The voltage across the capacitor is limited to a value approximately equal to that of the sum of the breakdown voltages of the MOV's. While the system of the '535 patent offers some improvement, it still suffers drawbacks. First, additional hardware including at least four thyristors and two MOV devices is required. Second, discharge of the capacitor is performed crudely and is dependent on the breakdown voltage of the MOV devices. Third, there is no mechanism for controlling the rate of capacitor discharge to minimize stress on TSC components.
Thus, the prior art is deficient in that it does not provide any mechanism for discharging the capacitor in a flexible and cost efficient manner so as to minimize the maximum voltage across the thyristor without requiring additional switching and clamping devices. It would be desirable to limit the voltage across the thyristor by completely discharging the capacitor so that the voltage across the thyristor is simply the source voltage. Moreover, it would be beneficial to perform that discharging in a flexible and controlled manner to permit both rapid and gradual rates of capacitor discharge depending upon system design objectives.